sysctl.conf(5): Mention sysctl.conf.local in the sysctl.conf(5) manual page

The possibility of using a sysctl.conf.local on a machine that has a shared
sysctl.conf(5) isn't documented. So mention the sysctl.conf.local in the
manual page.

PR: 254901
Submitted by: Jose Luis Duran <jlduran at gmail dot com>
Reported by: Jose Luis Duran <jlduran at gmail dot com>
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D29673

(cherry picked from commit 46c99528078ad478f50910110a933abef0b89cde)